Water-based latex paint is generally composed of an aqueous dispersion of pigments and latex particles that impart substrate hide, water resistance, and durability to the solid paint film. Other components such as dispersants, surfactants, and thickeners can be added to the liquid paint to maintain a stable dispersion and suspension of the pigments and latex particles. Solvents, bases, defoamers, and biocides can also be incorporated to improve liquid stability, application performance and film formation properties.
The chemical composition of water-based latex paints is designed to allow dispersion of components in water, yet maintain water resistance upon curing and forming a dry paint film. Essentially, the compositions are designed to contain a hydrophobic component for water resistance as a paint film, and a hydrophilic component to improve stability, solubility, and dispersion in the liquid aqueous phase.
Latex polymers are the film-forming portions of the paint film, and are prepared by an emulsion polymerization reaction. Aggregation of polymer particles is typically discouraged by including a stabilizing surfactant in the polymerization mix. In general, the growing latex particles are stabilized during emulsion polymerization by one or more surfactants such as an anionic or nonionic surfactant, or a mixture thereof. Examples of surfactants suitable for emulsion polymerization are given in McCutcheon's Detergents and Emulsifiers (MC Publishing Co., Glen Rock, N.J.) (2007 ed.). Generally, emulsion polymerization includes use of nonionic surfactants to create monomer micelles within the water phase.
Water-borne paint and coating formulations are among the most complex systems in surface and colloid chemistry. Surfactants are needed to reduce the free energy of the various interfaces of the system, thereby kinetically stabilizing these paint formulations. Commonly, surfactants are used as binder emulsifiers and as pigment dispersants. Additionally, surfactants are added to paint and coating formulations to improve wetting on low energy substrates, to control foaming during application and processing, and to prevent film defects caused by surface tension gradients. Handbook of Applied Surface and Colloid Chemistry, p. 105, Holmberg, K. (2001).
Nonionic surfactants, nonionic and anionic dispersants, nonionic thickeners, anionic alkali swellable thickeners, and water soluble cellulosic thickeners can be used in paint and coating compositions to separate, suspend and stabilize latex particles and pigment particles. Generally, the structures of these paint components contain a hydrophobic functionality synthetically combined with a hydrophilic functionality. Inorganic pigments are relatively heavy particles that would agglomerate and settle at the bottom of a container of latex paint without the use of anionic surfactants as well as various dispersants and thickeners for suspension.
Typically, surfactants, thickeners and dispersants are generally lower molecular weight components that remain in the paint film, which can significantly reduce water resistance and durability of the paint film. These components are required to maintain stability in the aqueous phase for in-can storage, but can compromise the end use function of a paint film.
In contrast to latex compositions, oil-based compositions, e.g., oil-based paints, commonly employ vegetable oils such as linseed oil or tung oil and/or vegetable oil co-reacted with other compounds (such as alkyd resins) as a component of the vehicle in the paint. The vegetable oils, which are also referred to in the art as “drying oils”, form crosslinked films upon exposure to air. Like all vegetable oils, these drying oils are triesters of various fatty acids and glycerol (i.e. “triglycerides” or “triacylglycerols”). However, unlike most vegetable oils, the fatty acids in drying oils have a very high degree of unsaturation (high iodine value), are high in polyunsaturated fatty acids, and generally have a majority of fatty acids that contain 3 or more double bonds (such as, for example linolenic [cis-9-cis-12-cis-15-Octadecatrienoic] acid, eleostearic [cis-9-trans-11-trans-13-Octadecatrienoic] acid, and 4-Oxo-cis-9-trans-11-trans-13-Octadecatrienoic acid). Semi-drying oils have moderate to high degrees of unsaturation, and are high in polyunsaturated fatty acids, but contain lower levels of fatty acids that have 3 or more double bonds.
The use of such reactive drying oils in oil-based paint helps to provide a paint film which is hard and durable. Thus, the drying oils and co-reacted vegetable oil products (alkyds) are desirable components of oil-based compositions. However, oil-based compositions typically contain large proportions of volatile organic compounds (“VOC's”) as solvents or additives, e.g., 380 to 450 grams per liter (“g/l”) or more. Such high concentrations of VOC's are environmentally undesirable.
Latex compositions, on the other hand, typically contain lower concentrations of VOC's, e.g. less than about 250 g/l and thus are more environmentally compatible. Accordingly, it would be desirable to incorporate the drying oils of oil-based compositions into latex compositions to promote crosslinking of the latex compositions. However, the drying oils used in oil-based compositions are not water-soluble and accordingly cannot readily be used in latex compositions.
It would also be desirable to develop a latex paint formulation which incorporates components that can react during the curing process, and thereby help form a durable, water-resistant paint film. It would further be desirable to reduce the amounts of the water-soluble or water-sensitive components which provide emulsifying and rheologic properties in the can but also can contribute to poorer properties of the dry coating.
U.S. Pat. No. 6,924,333 (“the '333 patent”) discloses a latex paint composition comprising polyunsaturated fatty acid-containing additives derived from vegetable oils. In one aspect, the '333 patent discloses a latex paint composition comprising a latex polymer and a polyunsaturated fatty acid moiety chemically attached to a glycol or a polyol through an ester, ether, or urethane linkage. The '333 patent further discloses that in a preferred embodiment, the glycol is ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, or 1,3-propane diol.
Although the compositions disclosed in the '333 patent may be suitable for particular latex compositions, it would be desirable to develop additional latex compositions having components that can react during the curing process and thereby help form a durable, water-resistant paint film.
Another component typically found in aqueous coating compositions, such as latex paints, is a humectant. A humectant is usually added to serve as both an antifreeze agent and to control the rate of evaporation from the coating film. Ethylene glycol is most commonly used, followed by propylene glycol.
As an antifreeze, the glycol depresses the freezing point of the aqueous medium. As water freezes, its volume expansion can push the latex particles together with sufficient force to cause coagulation by overcoming the stabilizing dispersant/surfactant layer. With a glycol added, even those paints or other coatings which become cold enough to freeze tend to freeze to a slush that exerts less force on the latex particles.
As a humectant, the glycol controls evaporation from the paint film to facilitate wet lapping. When paint is applied by roller or brush, each brush-full or roller-full of paint overlaps the wet edge of the preceding area of coverage. Without the glycol, evaporation can quickly leave the previously applied film edge very viscous, although very weak due to only limited latex coalescence. The brush or roller shear from the subsequent lapped application can break up this film, leaving irregularities along the lapped edge (Ciullo, P. A., Industrial Minerals and Their Uses—A Handbook and Formulary William Andrew Publishing/Noyes (1996)).
Although ethylene glycol and propylene glycol are suitable humectants for latex paints and other coatings, they contribute to VOC's. As VOC's are generally considered to be environmentally detrimental, it would be highly desirable to substitute ethylene glycol and propylene glycol at least in part with lower- or zero-VOC humectants in aqueous coating compositions.